Microresonator-based high-performance high-pressure sensor and system

ABSTRACT

An optically-powered integrated microstructure pressure sensing system for sensing pressure within a cavity. the pressure sensing system comprises a pressure sensor having an optical resonant structure subject to the pressure within the cavity and having physical properties changing due to changing pressures within the cavity. A substrate supports the optical resonant structure. An input optical pathway evanescently couples light into the optical resonant structure. An output optical pathway collects light from the optical resonance structure. A light source delivers a known light input into the input optical pathway whereby the known light input is evanescently coupled into the optical resonant structure by the input optical pathway and a portion of such light is collected from the optical resonant structure by the output optical pathway. A light detector receives the portion of the light collected from the optical resonant structure, and generates a light signal indicative of such portion of the light collected from the optical resonant structure. A temperature compensation sensor generates a temperature signal indicative of the temperature near the optical resonant structure. A spectrum detection device receives the light signal and temperature signal. The spectrum detection device analyzing the light signal and the temperature signal with a detection algorithm to generating a pressure signal indicative of the pressure within the cavity.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent application claims priority to the provisional patentapplication identified by U.S. Ser. No. 60/548,046 filed on Feb. 26,2004, the entire content of which is hereby incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

The present invention was supported by National Institute of Science andTechnology (NIST) Small Business Innovation Research (SBIR) program,SB1341-04-W-1128.

BACKGROUND OF THE INVENTION

High-pressure transducers play several important roles in a variety ofindustries. The most demanding application for precision high-pressuretransducers may be found in the oil and gas production industry. The oiland gas production capacity of a reservoir is determined from modelsusing the precision measurement of pressure during and after product isvented through a standard orifice. Monitors for reservoir pressureoperate in a “downhole” environment with pressures up to 200 MPa (25,000psig) and temperatures up to 300° C. This measurement is critical toenergy production companies since it directly influences the ability tofinance operations.

Additional applications for high-pressure transducers include monitoringwater jet cutting equipment, plastic extrusion, and hydroformingprocesses. Operating pressures for water jet cutting machines may exceed413 MPa (60,000 psig). Current pressure monitors utilize straingauge-based devices. The accuracy of these gauges rarely exceeds 0.01%.Hydroforming is a machining technique that uses high pressure to forcethe work material onto a die. Proper operation of the hydroformerrequires high dynamic monitoring of extreme pressures.

Quartz pressure transducers, which have earned their reputation in highstandard pressure sensing, have been widely used for some of thosecrucial applications. The high accuracy and stability of the quartzthickness shear mode resonance (TSMR) have been used as the high qualityfrequency standard sensing technique, along with surface acoustic wave(SAW) devices1. The advantages of quartz pressure sensors are: goodpressure range (up to 280 MPa), high resolution (1 ppm [parts permillion] or 0.0001%), lower susceptibility to environmental parameterssuch as temperature (with special cutting), and long-term stabilitywithin a protected environment. The resolution and accuracy come fromthe high resonance quality (Q factor). The quartz Q factor can be ashigh as 4000, which needs to be carefully preserved in a hermeticallysealed, evacuated environment.

However, there are several problems with quartz pressure sensingtechnologies that trigger motivation to seek new methods to addressthese needs. Quartz pressure transducers appear to be limited forapplications by higher pressure due to mechanical “twinning” of thepressure sensitive crystal element. “Twinning” is the reversal of thepiezoelectric polarity under stress. For a crystal oscillator, the onsetof twinning stops the crystal oscillation. For applications above 280MPa, new materials and methods must be used.

As new techniques have been introduced in the oil and gas industry, newrequirements have pushed quartz pressure transducers to the limits oftheir capabilities. Horizontal production techniques, which allow theextension of the production zone, mandate the use of smaller pressuretransducers. Current transducers are typically larger than one inch indiameter. A target diameter of one-half inch or less is desirable tosupport horizontal operations; such size reductions are difficult forquartz technology to achieve.

The quartz pressure transducer technology dates from the middle 1960s.Since that time, crystal manufacturing operations have been migratingoverseas to minimize labor expenses. Crystal manufacturers thatpreviously provided the special purpose elements used in the quartzsensors as an adjunct to the more volume-oriented frequency controlbusiness have been severely and negatively impacted by this migration.The development expense is increased by the reluctance of crystalmanufacturers to participate in specialty crystal design given theminimal potential market. The lifetime of quartz pressure transducerswas, perhaps, underestimated since many existing tools date from theoriginal production. Given the aging fleet of existing sensors, anopportunity exists to infuse a new and more “manufacturable” technology.

Additionally, the quartz reference and sensors along with the necessaryelectronics package must operate in harsh environments for long periodsof time (e.g., several years) with a minimum of maintenance. The severeenvironment dictates that extensive efforts must be made to protect theequipment. These measures contribute to the expense of the transducer.Transducer calibration also must be certified for temperature andpressure. The certification process is time consuming and adds to thetransducer cost.

In natural quartz, the left and right forms are about evenlydistributed, resulting in optically twinned material. The presence oftwinning prevents the crystal from being used as a resonator. On theother hand, cultured quartz is mostly right forms of quartz, so thattwinning is not a problem. However, under very high pressure, severaldeformation mechanisms, such as mechanical twinning and creeping, mayoccur in the crystal lattice structure. These pressure-induced defectscause the failure of those quartz-based pressure sensors. This fact setsthe upper limit of quartz pressure sensing to about 140-280 MPa.

Finally, yet importantly, it is almost impossible for the quartztransducer to perform direct sensing without the translation of a forcecollector such as a bellows or Bourdon tube because quartz has notolerance to the surface contamination. The introduction of a forcecollector not only injects hysteresis and noise but also complicates thedesign.

High quality factor mode (HQM) micro-resonator technology is a uniqueoptical resonance phenomenon with extremely high resonance qualityfactor (Q factor can be ˜1E10 or 10¹⁰), which inherently enables itselfin the ultra-high resolution spectroscopy. HQM micro-resonators aremostly made of fused silica or glass material, which does not sufferlattice defects under significant pressure. The pressure sensitivity issimilar to or better than quartz while it can maintain elasticity up to9 GPa.

These micro-resonators can be realized in the form of micro-spheres,micro-cylinders, or even a micro-ring/disk structures embedded in anoptical chip. Their diameters typically are as small as 5 μM or up to amillimeter, but most are around 100 μm or less.

The following list summarizes some of the technical merits of HQMmicroresonator-based pressure sensing technology:

-   -   Ultra-high resolution technology    -   Capability to sustain pressure above 500 MPa    -   Microscopic sensing element    -   Completely passive device    -   Optical interrogation

Researchers have realized since the days of Lord Rayleigh thatdielectric materials can be used as waveguides and optical resonators.One of the famous HQM resonance is the so-called whispering-gallery mode(WGM) resonance of spherical dielectric particles, which was studied indetail nearly sixty years ago. A microsphere is essentially a fusedquartz ball (typically on the order of 100 μm more or less in diameter).Since the sphere is generally more optically dense than its surroundingmedium, light in the sphere can be internally reflected. Lightpropagating inside the sphere would then be spatially constrained totravel along the perimeter of a great circle of the sphere (theperimeter of a plane that intersects the sphere with maximum area). Thelight propagates inside the sphere until it is absorbed or scattered bymaterial imperfections. Light constrained in this way is said to betrapped in a whispering-gallery mode. WGM resonators have been proposedfor several applications, such as add/drop filters in opticalcommunication, optical switches, laser cavities, and high-resolutionspectrometers.

Excitation and interrogation of the HQM's can be accomplished byevanescent-wave coupling. Input and output coupling can be achieved byoverlapping the HQM's evanescent field with that of a prism oreroded/angle-polished/tapered single-mode optical fiber by way ofexample, but not limitation. FIG. 2 shows a waveguide coupling lightinto the microring resonator from its left side.

Braginsky et al. pointed out several years ago that the low losses andsmall electromagnetic mode volumes of HQM make high-Q microresonatorsattainable. A resonance quality (O) as high as 8E9 has been observed inthe laboratory environment and Q up to 1E9 can be preserved in protectedenvironments (such as hermetically sealed boxes) for a long period. Dueto the nature of high-Q resonance, the spectral peaks (or nulls) of theresonance spectrum are very narrow, which essentially provides thecapability of very narrow-band optical filtering.

The outstanding resonance quality of HQM's can be directly translated tohigh measurement resolution and stability. The spectral resolution canbe derived by the Q factor and amplitude resolution as follow. Thefull-width at half-maximum (FWHM) can be expressed as$\frac{\lambda}{Q}.$If the resonance peak is roughly modeled as a triangular shape, then thespectral resolution can be derived as the multiplication of amplituderesolution and FWHM. For example, the FWHM is 1.55 pm (picometer) for aQ=1E7 resonator at 1550 nm. With 1% amplitude resolution, the spectralresolution will be 0.0015 pm. This number is based on the spectral shiftmeasurement without going into interferometer design.

Fiber Bragg grating (FBG) is one widely used sensing technology used tooptically measure pressure or force-induced strain. However, due to itslimited Q factor (˜1E4 or lower) it is difficult to provide enoughmeasurement resolution because of its broad spectral feature. SeveralFBG-based pressure sensing studies have been published. Xu et al. hasreported a FBG sensor with 0.22-nm spectral shift under 70-MPa fordirect sensing. The pressure sensitivity is about 3 pm/MPa. Otherresearch has used special side-hole FBG and boosted the sensitivityabout two times. A FBG-based pressure sensor commercialized by Sabeushas listed resolution of 0.05%, which is consistent with our analysis.With limited pressure sensitivity and spectral resolution, the FBG isless appealing in the high-resolution pressure sensing.

Though FBG technology may not be suitable to for high-resolutionpressure sensors, many research and development efforts in temperaturecompensation and signal interrogation can be translated into HQMresonator technology because of their common nature in spectral domaininterrogation and temperature compensation.

Currently, quartz transducers typically employ specialized quartzsensors. A typical quartz pressure transducer would include a referenceoscillator, using a temperature- and stress-compensated crystal, aquartz temperature sensor, and a quartz pressure sensor. The quartztemperature sensor provides a temperature measurement independent ofpressure so that the pressure measurement may be compensated fortemperature variations. The quartz pressure sensor is designed for aspecific response to stress applied in a plane determined by a quartzforce collector. The pressure sensor is exposed to external pressurewhile the electronics package, temperature, and reference elements areisolated.

The HQM resonances will not only respond to the strain but also to thetemperature. Xu et al. first reported a discrimination technique byusing superimposed dual FBGs at 850 and 1300 nm. Afterward, manyapproaches were reported by using dual FBG with different fibermaterials, diameters, and grating types. These methods are all based onthe differentiation of strain and temperature effect upon spectralshift. The spectral shift of the two FBG wavelengths (Δλ_(1,2)) can bemodeled as follow: ${\begin{bmatrix}{\Delta\lambda}_{1} \\{\Delta\lambda}_{2}\end{bmatrix} = {\begin{bmatrix}K_{ɛ1} & K_{T1} \\K_{ɛ2} & K_{T2}\end{bmatrix}\begin{bmatrix}{\Delta ɛ} \\{\Delta\quad T}\end{bmatrix}}},$

-   -   where K_(ε1,2) are the strain-response coefficients, K_(T1,2)        are the thermo-response coefficients, Δε is the applied strain        due to pressure, and ΔT is the temperature change. The        contribution due to pressure-induced stress is introduced        through Young's modulus, Poisson ratio, and photoelastic        constant, while the contribution from temperature is determined        by thermal expansion and thermo-optic coefficients. Therefore,        the coefficient ratio $\frac{K_{ɛ1}}{K_{ɛ2}}$        should be different from $\frac{K_{T1}}{K_{T2}}.$        This discrepancy provides information to solve Δε and ΔT,        respectively.

Thus, a need exists for an improved pressure sensing system which has apressure sensor which can be made smaller in size and capable ofaccurately measuring higher pressures than the quartz pressuretransducers discussed above. It is to such an improved pressure sensingsystem that the present invention is directed.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a schematic view of an optically-powered integratedmicrostructure pressure sensing system, constructed in accordance withthe present invention, for sensing pressure.

FIG. 2 is a diagram illustrating a waveguide-coupled high quality factormode of a resonant light propagating within a microring optical resonantstructure.

FIG. 3 is a chart illustrating a null spectrum of a whispering gallerymode detected at a through port.

FIG. 4 is a chart illustrating the high quality factor mode spectrumdetected at a drop port of a micro-ring resonator, the layout of themicro-ring resonator and the waveguide forming the drop port is shown inthe inset.

FIG. 5 is a chart illustrating a high quality factor mode of spectralshift versus a weight load applied to the optical resonant structure.

FIG. 6 is a perspective view of a pressure sensor constructed inaccordance with the present invention configured as an optical chiphaving a micro-ring resonator forming the optical resonant structure.

FIG. 7 is a partial schematic, partial cross-sectional view of apressure sensor connected to a vessel by way of a spiral tubing fordetecting the pressure within the vessel.

FIG. 8 is a schematic view of a data acquisition set up for testingand/or calibrating the pressure sensing system.

FIG. 9 is a perspective view of a pressure sensor, constructed inaccordance with the present invention, for sensing a differentialpressure between two vessels.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, and in particular to FIG. 1, showntherein and designated by a reference numeral 10 is an optically-poweredintegrated microstructure pressure sensing system for sensing pressurewithin a vessel 12, such as a flow-line or tank. The optically-poweredintegrated microstructure pressure sensing system 10 will be referred tohereinafter as the “pressure sensing system”. An example of the vessel12 is depicted in FIG. 7. In general, the pressure sensing system 10 isprovided with one or more pressure sensor 14, one or more light source16, one or more light detector 18, one or more temperature compensationsensor 20, and one or more spectrum detection device 22.

The pressure sensor 14 is provided with one or more optical resonantstructure 30, one or more substrate 32, one or more input opticalpathway 34, and one or more output optical pathway 36. The opticalresonant structure 30 is subject to the pressure within the vessel 12and has physical properties changing due to changing pressures withinthe vessel 12. The substrate 32 supports the optical resonant structure30. The input optical pathway 34 is positioned to evanescently couplelight transmitted through the input optical pathway 34 into the opticalresonant structure 30. The output optical pathway 36 is positioned tocollect light from the optical resonant structure 30.

The light source 16 delivers a known light input into the input opticalpathway 34 whereby the known light input is evanescently coupled intothe optical resonant structure 30 by the input optical pathway 34. Aportion of the light evanescently coupled into the optical resonantstructure 30 is collected from the optical resonant structure 30 by theoutput optical pathway 36. The light detector 18 receives the portion ofthe light collected from the optical resonant structure 30 by the outputoptical pathway 36. The light detector 18 generates a light signalindicative of such portion of the light collected from the opticalresonant structure 30. The temperature compensation sensor 20 generatesa temperature signal indicative of the temperature near the opticalresonant structure 30 so that temperature compensation of the pressuredetected by the pressure sensor 14 can be effected. The spectrumdetection device 22 receives the light signal and the temperaturesignal. The spectrum detection device 22 analyzes the light signal andthe temperature signal with a detection algorithm to generate a pressuresignal indicative of the pressure within the vessel 12.

When the known light input in the input optical pathway 34 goes throughthe optical resonant structure 30 as shown in FIG. 2, some energy at theresonance wavelength gets trapped inside the optical resonant structure30. In one preferred embodiment, the input optical pathway 34 and theoutput optical pathway 36 are formed by a single waveguide, such as astrand of optical fiber, extending past the optical resonant structure30. In this instance, the output optical pathway 36 can be referred toas a “through port.” The spectrum at the “through port” has a so-called“null spectrum.” FIG. 3 shows the null spectrum of a 125-μm WGMmicro-cylinder. At the bottom, the arrows indicate the mode order: thelongest arrows stand for the lowest order mode, while the second longestarrows for the 2^(nd) order (radial) mode. As the arrows get shorter,the mode number is higher. The spectral spacing between two adjacentlowest order modes is the free spectral range (FSR). A similar spectralfeature will be repeated every FSR. The HQM spectrum will shift understrain, but the amount of shift may be different for each order ofresonance.

In another preferred embodiment, the input optical pathway 34 and theoutput optical pathway 36 are formed of separate waveguides. In thisinstance, the output optical pathway 36 can be referred to as a “dropport.” The HQM spectrum at the drop port will be a “peak spectrum”instead of the “null spectrum” as shown in FIG. 4. The input opticalpathway 34 and/or the output optical pathway 36 can be formed by planarwaveguide(s) approaching the side or top of the optical resonantstructure 30, which would have a nearly permanent relationship with theoptical resonant structure 30.

The optical resonant structure 30 is a completely passive device. Theonly requirement for the operation of the optical resonant structure 30is a waveguide connection, such as the input and output optical pathways34 and 36 which provide optical interrogation signals to perform thesensing. The known light signal is substantially immune to theelectromagnetic interference (EMI) from the sensing environment.Furthermore, the spectral domain data acquisition enables wave divisionmultiplexing (WDM) to be implemented. Thus, the spectrum detectiondevice 22 can include a centralized readout system away from the harshsensing environment and can support multiple pressure sensors 14 fordistributed sensing. This feature can reduce the average cost ofindividual pressure sensors 14, as well as the cost of operation andmanagement dramatically.

The optical resonant structure 30 can be a WGM resonator. WhisperingGallery mode (WGM) resonance describes the phenomenon whichelectromagnetic (EM) energy, such as light, can be trapped inside acavity with a smooth circular boundary and propagating around theperimeter through successive total internal reflection from the cavityboundary. The refractive index of the cavity needs to be greater thanthat of its ambient material so that the total internal reflection issupported and WGM resonance is possible. The propagation of the EMenergy of the WGM mode is confined by a single interface formed by thecavity and its ambient material. The EM field intensity outside ofcavity decays exponentially along the radial direction. This type offield is termed “evanescent field”. On the other side, the EM fieldinside the cavity is not confined by any physical boundary. Therefore,many different field distributions can be supported by this structure.All of the field distributions have only a single evanescent wave tailextending outside of the cavity/ambient material interface.

The WGM resonance may be found in several forms of structures: sphere,cylinder, and disk. Their structures are in solid volume and require atleast one plane with perfect circular perimeters to support WGMresonance. The circular perimeter is essential criteria for theproduction of WGM resonance.

The common means of coupling EM energy into and out of the WGM resonanceis through the evanescent field coupling. When the evanescent field ofthe coupler overlaps with the evanescent field of the WGM resonance, theEM energy can be transferred between the coupler and the WGM resonator.

Because the cavity material usually has very low optical loss and boththe total internal reflection and evanescent field coupling are low lossprocesses, the WGM resonator can preserve the trapped EM energy veryefficiently. When the WGM resonator has very low loss, the resonancequality factor becomes high and the resonance spectrum has very narrowbandwidth, which is suitable for high resolution sensing applications.

The optical resonant structure 30 can also be a closed-loop waveguideresonator. Optical waveguides were designed to confine the EM energy andguide the energy propagating toward a desired direction. They usuallyhave two or more interfaces formed by the waveguide material and theirambient material, which support multiple evanescent fields. Because oftheir low optical loss material and capability to confine the energyinside the waveguide, optical waveguide, which is not a resonantorinitially, can preserve EM energy very efficiently as long as the energyis propagating inside the waveguide. By connecting the two ends of anoptical waveguide, an optical resonator is formed by this close-loopwaveguide. The energy preserving capability of the optical waveguidedoes not depend on the route of the waveguide at all. An opticalwaveguide can be arranged in a linear route, an irregular route, or acircular loop without changing its waveguiding property significantly.This is different from a WGM resonator which requires a circularboundary to form the WGM.

An optical waveguide can be connected with its two ends to form acomplete loop, which is actually a resonant structure. This closed-loopwaveguide (CLW) resonator provides an infinite long path to trap the EMenergy propagating inside the loop with great efficiency. This loop mayhave any shape without changing its capability to preserve the EMenergy.

By combining the excellent energy preserving property of the close-loopwaveguide with the commonly used evanescent field coupling technique,the close-loop waveguide can be used as an innovative structure tosupport high quality factor mode (HQM) resonance. Both WGM resonatorsand CLW resonators can support HQM resonance, but they are based oncomplete different physical phenomena.

Since the CLW resonators have multiple material interfaces, there aremultiple evanescent wave tails extended out of all interfaces. With thislarge amount of evanescent field, the sensitivity of the sensingapplications that rely on the evanescent fields can be greatly improved.

Even though the shape of the loop is not essential for the CLWresonators, the circular loop has the lowest bending loss as the size ofthe loop reduces to the micron range. Micro-ring is one example of theCLW resonators at micrometer size. When the size of the closed-loopwaveguide resonator shrinks down, the circular shape of the closed-loopwaveguide has minimum loss. However, circular closed-loop waveguideresonators are still not WGM resonators.

The HQM resonance is a parametric resonance phenomenon, which isdependent on the material, property and geometry of the optical resonantstructure 30, as well as the ambient environment surrounding the opticalresonant structure 30. The pressure applied on the optical resonantstructure 30 produces elastic geometric changes and refractive indexchanges, which in turn cause a WGM spectral shift as shown in FIG. 5.

Besides their advantage in spectral resolution, HQM-based sensors alsohave important material advantages. The optical resonant structure 30 ispreferably fabricated of glass material, also sometimes referred to asamorphous silica or fused quartz. Defects in the lattice structure donot present in this material. The HQM resonance is an optical parametricresonance, which typically does not suffer catastrophic failure due tothe material property change.

The behavior of amorphous silica under extremely high pressure (e.g.,several GPa) has been studied. Experiments show that the bulk moduluswill decrease with increasing pressure to a minimum at −3 GPa. However,this effect remains reversible until 9 GPa has been reached. Therefore,the glass material should be a good candidate for pressure sensing in arange from about 280 to about 500 MPa.

The optical resonant structure 30 can be constructed of a microsphere,or other type of resonant structure, such as a micro-cylinder, amicro-disk, and a micro-ring resonators. Microspheres andmicro-cylinders often are fabricated as discrete components. Thefabrication process usually generates very smooth surfaces due to thesurface tension and high heat, which makes a Q factor ranging from1E6-1E8 possible. However, surface cleanness and coupling alignment needto be carefully maintained to preserve the HQM resonance. Fortunately,micro-disks and micro-ring resonators are often realized as a planaroptical structure. Light can be coupled in and out via planar waveguides(input optical pathway 34 and/or output optical pathway 36) approachingthe side or top of the optical resonant structure 30, which has a nearlypermanent relationship with the optical resonant structure 30. As willbe described in more detail below, in one preferred embodiment theoptical resonant structure 30 can be sandwiched by the substrate 32 anda cladding 40 (FIG. 6) to form an optical chip 44 for use in forming thepressure sensor 14, which provides excellent protection to thestructures and resonance field. The substrate 32 can be constructed ofany rigid material that is optically transparent at the operatingwavelength, such as silica, glass material or optical quality polymermaterial. In this way, the high-Q HQM resonance can be stabilized for along time. The problem of a planar optical resonant structure 30 is thelimited Q factor (<5E5) due to its rough surface through fabrication bylithography or etching. The cladding 40 can be constructed of any typeof material capable of transmitting pressure to the optical resonantstructure 30. As shown in FIG. 7, the cladding 40 can be constructed ofa fluid surrounding at least a portion of the optical resonant structure30. To form the optical chip 44, the cladding 40 can also be constructedof a solid coating, and in this instance, the cladding 40 is desirablyconstructed of an elastic optically transparent material, such asplastic, latex rubber, or polymer material. The same substrate materialcan be used as cladding material. The glass material can be used asharder cladding and the polymer-based material as softer cladding.

Referring again to FIG. 1, the light source 16 can be any type of lightsource (either broadband or narrow band) which produces a known orpredetermined light signal. For example, the light source 16 can beconstructed with a scanning tunable diode laser (TDL). A suitable TDLhas been found to be a model TDL 6328 obtainable from New Focus, Inc. ofSan Jose, Calif. The New Focus TDL 6328 has a spectral range of1519-1576 nm with short-term bandwidth of 300 kHz and long-termbandwidth of 5 MHz. The smallest spectral step size is about 0.3 pm.

The light detector 18 can be a photodiode, CVD, phototransistor or anyother device capable of detecting the light in the output opticalpathway 36 so that the optical spectrum in the output optical pathway 36can be sampled or recorded. For example, the light detector 18 can be anIR detector. For the narrow-band light sources such as a laser, thelight detector 18 can be a photodiode to acquire the spectrum. For thebroad-band light sources such as light emitting diode, amplifiedspontaneous emission (ASE), or any white light sources, the lightdetector 18 can be an optical spectrometer with appropriate spectralresolution to acquire the spectrum.

The light source 16 can be calibrated with a multi-wavelength meter. Asuitable multi-wavelength meter is an HP 86120B multi-wavelength meterwith 1-pm resolution obtainable from Hewlett-Packard Co. of Palo Alto,Calif. The light detector 18 can also include an optical power meter formeasuring throughput intensity. A suitable optical power meter, e.g., amodel 1825-C, can be obtained from Newport Corp. of Irvine, Calif. Byscanning the wavelength of the light source 16 and recording the powerreading by the light detector 18, the HQM spectrum can be acquired. Thelight detector 18 can include a high-resolution optical spectrumanalyzer to speed up the spectrum acquisition process. In one preferredembodiment, the input optical pathway 34 and the output optical pathway36 include high quality angle-polished single-mode fibers to removeundesired light intensity fluctuation due to the etalon effect. Thecurrently demonstrated 3-pm resolution is limited by the in-housewavelength measurement instrument used in the calibration process.

The temperature compensation sensor 20 can be any type of temperaturesensor, such as a thermistor, a thermocouple, a resistance temperaturedetector, a semiconductor thermometer device, or a thermal imagingdevice. One preferred embodiment of an optically powered temperaturecompensation sensor 20 will be described in more detail below.

The spectrum detection device 22 includes a computer or computer systemfor receiving the light signal and the temperature signal. The computeror computer system runs the detection algorithm to generate the pressuresignal. The terms “computer system” and “computer” as used herein mean asystem or systems which are able to embody and/or execute the logic ofthe processes described herein. The logic embodied in the form ofsoftware instructions or firmware may be executed on any appropriatehardware which can be a dedicated computer system or systems, a personalcomputer system, a main frame computer system, a mini-computer system, aserver, or a distributed processing computer system. All of thesecomputer systems are well understood in the art. Thus, a detaileddescription of how to make or use such computer systems is not deemednecessary herein. In one preferred embodiment, the spectrum detectiondevice 22 also includes software to automate the operation of the lightsource 16, light detector 18 and temperature compensation sensor 20 toobtain the data necessary to form the pressure signal. For example, thedata acquisition process can be automated by a personal computer runninga Labview program(s).

FIG. 7 illustrates one embodiment of the pressure sensor 14 formeasuring the pressure within the vessel 12. The pressure sensor 14includes a pressure-sensing chip 52 or the optical chip 44 within apressure vessel 54. The pressure sensing chip 52 includes the substrate32, the optical resonant structure 30, the input optical pathway 34 andthe output optical pathway 36. The input optical pathway 34 and theoutput optical pathway 36 are connected to optical fibers 55 sealed witha high pressure feed-through 60. The temperature compensation sensor 20comprises an optical resonant structure 62, an input optical pathway 64,and an output optical pathway 66. The optical resonant structure 62 ismounted on the substrate 32 (or another substrate) and covered with arigid cladding 68 so as to have different pressure response from thestructure 30 within the pressure vessel 54. The input optical pathway 64is positioned to evanescently couple light transmitted through the inputoptical pathway 64 into the optical resonant structure 62 of thetemperature compensation sensor 20. The output optical pathway 66 ispositioned to collect light from the optical resonance structure 62 ofthe temperature compensation sensor 20. The pressure sensing system 10preferably includes an optical splitter (not shown) for directing theknown light signal generated by the light source 16 to the input opticalpathways 34 and 64 of the pressure sensor 14 and the temperaturecompensation sensor 20. The optical resonant structure 62, the inputoptical pathway 64 and the output optical pathway 66 can be constructedin similar manners as the optical resonant structure 30, input opticalpathway 34 and the output optical pathway 36.

The pressure vessel 54 of the pressure sensor 14 defines a port 70. Aspiral tubing 72 is provided for connecting the port 70 of the pressuresensor's pressure vessel 54 with a cavity within the vessel 12. Thepressure vessel 54 and the spiral tubing 72 are filled with a fluid 76such that the pressure-sensing chip 52 is immersed within the fluid 76.The fluid 76 forms the cladding 40, and also serves to transmit pressurewithin the vessel 12 to the optical resonant structure 30 of thepressure-sensing chip 52 while also isolating the pressure sensing chip52 from the contents of the vessel 12.

The detection algorithm desirably uses a discrimination technique fordetecting the pressure within the vessel 12 using the light signal andthe temperature signal. This method is based on the differentiation ofstrain and temperature effect upon spectral shift. The spectral shift ofthe wavelengths (Δλ_(1,2)) from the optical resonant structure 30 of thepressure sensor 14 and the optical resonant structure 62 of thetemperature compensation sensor 20 can be modeled as follows:$\begin{matrix}{{\begin{bmatrix}{\Delta\lambda}_{1} \\{\Delta\lambda}_{2}\end{bmatrix} = {\begin{bmatrix}K_{ɛ1} & K_{T1} \\K_{ɛ2} & K_{T2}\end{bmatrix}\begin{bmatrix}{\Delta ɛ} \\{\Delta\quad T}\end{bmatrix}}},} & {{Eq}.\quad(1)}\end{matrix}$

-   -   where K_(κ1,2) are the strain-response coefficients, K_(T1,2)        are the thermo-response coefficients, Δε is the applied strain,        and ΔT is the temperature change. The contribution due to stress        is introduced through Young's modulus, Poisson ratio, and        photoelastic constant, while the contribution from temperature        is determined by thermal expansion and thermo-optic        coefficients. Therefore, the coefficient ratio        $\frac{K_{ɛ1}}{K_{ɛ2}}$        should be different from $\frac{K_{T1}}{K_{T2}}.$        This discrepancy provides information to solve Δε and ΔT,        respectively. One extreme example is to expose the temperature        compensation sensor 20 only to temperature change but not to        pressure change while structure 30 is receiving both pressure        and same temperature change. This method essentially nullifies        the pressure-response coefficient (i.e., K_(ε11)=0), while        maintaining K_(ε2) and K_(T2). Then, the Eq. (1) can be used to        derive both pressure and temperature changes. Additionally,        various cladding material can be used to form cladding 40, which        will render different temperature and pressure response from        structure 30 for the temperature and pressure response        discrimination. Finally, micro-spheres and micro-disks        inherently have strong high order resonance spectra, which have        different responses to pressure and temperature. This unique        characteristic allows HQM sensors to implement Brady's method        naturally.

Although the pressure sensor 14 has been shown and described utilizingthe spiral tubing 72 and the fluid 76 to indirectly transmit pressure tothe optical resonant structure 30, the pressure sensor 14 can bemodified to perform direct sensing by constructing the substrate 32 ofan optically transparent material, such as a glass material, andreplacing the fluid 76 forming the cladding 40 with a solid materialforming the cladding 40. For example, the cladding 40 can be constructedof a glass material or softer polymer material, and the entire pressuresensing chip 52 can be directly installed in the pressurized environmentto perform direct sensing. This can provide significant technical meritcompared to indirect sensing technologies such as quartz. Not only canthe mechanical hysteresis be substantially eliminated, but alsosubstantially all the mechanical noise and design complexity of theforce collector can be avoided. The net effect is better accuracy,repeatability, and stability.

The optical resonant structure 30, such as HQM micro-ring resonators,are commercially available from Little Optics of Annapolis Junction,Maryland and such micro-ring resonators are fabricated from a glassmaterial sold under the trademark “Hydex”, which has a refractive indexof about 1.7 and can achieve a refractive index contrast up to about25%. Hydex material and silica share similar mechanical properties suchas the capability of sustaining high pressure, temperature, andcompatibility to the existing lithography fabrication process. Thelayout of a micro-ring resonator is shown in the inset of FIG. 4. Thering configuration removes all the higher order resonance so that thereis only one resonance peak in each FSR. The double peaks are due to thetwo perpendicular polarizations. Although this is an off-the-shelfdevice with a Q factor of only about 5,000 (which is only suitable forits original design goal), it still demonstrates pressure sensingapplications. The intrinsic Q is 5E6, which sets the upper limit for theattainable Q after signal coupling.

As discussed above, the substrate 32 can be constructed of silica. Theoptical resonant structure 30 can be protected by the substrate 32 andthe cladding 40. As mentioned before, the material used as the substrate32 can also be used as the cladding 40. Therefore, force can be directlyapplied onto the substrate 32 and cladding 40 without damaging theoptical resonant structure 30. The input and output optical pathways 34and 36 can be pigtailed to standard telecom single-mode fiber, which ishandy for optical interrogation.

FIG. 5 shows the experimental data of the pressure induced HQM spectralshifts for two different cladding materials. As shown in FIG. 5, thewavelength red shifting effect (positive spectral shifting uponpressure) of microring with the fluid cladding is much faster than theblue shift (negative spectral shifting upon pressure) produced by themicroring with SiO₂ cladding. The pressure sensitivities for variousconfigurations and resonance modes are noted in the figure. It has beenconfirmed that the “fluid cladding”, where the fluid is the cladding 40,is more compressible than SiO₂ and the strain-optical effect of thefluid surpasses the size-changing effect of the optical resonantstructure 30 itself. The net effect is a much larger red shifting of thespectrum upon pressurization. The pressure sensitivities up to +118pm/MPa (811 fm/psi) using the silicon oil and +89 pm/MPa (613 fm/psi)using the Spinesstic 22 have been observed, which is almost two ordersof the magnitude over the sensitivity of the optical resonant structure30 with SiO₂ cladding. Pressure sensing resolution can be measured byusing the calibrated instrument such as the Ruska dead weight tester.

The pressure sensitivity and the resolution was tested by the followingprocedures:

-   -   1. The dead weight tester was parked at a calibrated static        pressure.    -   2. A small weight was positioned on top of the tester piston to        introduce a known calibrated pressure jump at a known time.        Then, the weight may be taken off the piston to produce a        pressure drop at another time.

3. A portion of the resonance spectrum was recorded continuously duringthe pressure jump/drop introduction events.

4. The acquired spectral data were processed to extract pressure-relatedspectral shift and determine system noise level. The pressuresensitivity can be found from the spectral shift and the resolution canbe derived from the sensitivity and the standard deviation (std) of thenoise through the following equation: $\begin{matrix}{{{resolution} = {\frac{{{std}({noise})} \times 2}{{pressure}\quad{induced}\quad{shift}} \times {pressure}\quad{j{ump}}\text{/}{drop}}},} & (2)\end{matrix}$which uses the twice the amount of the standard deviation of the signalduring the static pressure period as the measure to define the sensingresolution. Preliminary experiment results have demonstrated thatpressure resolution of 2 ppm over the full scale is possible.

Shown in FIG. 8 is a version of a calibration system 100 constructed inaccordance with the present invention for calibrating the pressuresensor 14. The calibration system 100 is provided with a light source102 (shown as a near infrared (NR) tunable diode laser), a coupler 104,a wavelength meter 106, a light detector 108 (shown as an IR detector),a power meter 110, a breakout box 1112, a temperature control box 114, acomputer 116, and a pressurizer 118 (shown as a screw pump/dead weighttester). The construction of the light source 102 and the light detector108 is similar to the construction of the light source 16 and the lightdetector 18 discussed above.

The pressure vessel 54 can be be filled with pressure fluid such as Dow200 silicone oil, Spinesstic 22, or Dioctyl Sebacate (DOS) fortransmitting the pressure to the optical resonant structure 30 of thepressure sensor 14. The pressurizer 118 pressurizes the pressure vessel54 up to a predetermined pressure of 40 MPa, for example. TheTemperature control box 114 is used for maintaining the temperature ofthe pressure fluid at a constant predetermined temperature.

In use, the pressurizer 118 pressurizes the vessel 54 to a variety ofpredetermined stepped pressures while the light source 102 feeds theknown light signal to the pressure sensor 114 via the input opticalpathway 34. The pressure within the vessel 54 can be determined by wayof a pressure gauge, preferably having a pressure resolution of 0.01%.The light signals generated by the optical resonant structure 30 arereceived by the light detector 108 and are fed to the computer 116 viathe power meter 110 and the breakout box 112. The breakout box 112provides the external connectivity for the data acquisition hardware toreceive signals from various sources. The computer 116 records the lightsignals and utilizes the light signals to calibrate the pressure sensor14 based on the known temperature and known pressures.

The light source 102 can be a New Focus TDL 6328 having a spectral rangeof 1519-1576 nm with short-term bandwidth of 300 kHz and long-termbandwidth of 5 MHz. The smallest spectral step size is about 0.3 pm. Thewavelength meter 106 can be a model HP 86120B multi-wavelength meterwith 1-pm resolution which is used to provide wavelength calibration.The light detector 108 can be a Newport optical power meter 1825-C tomeasure throughput intensity. By scanning the TDL wavelength andrecording the power reading, the HQM spectrum can be acquired. Ahigh-resolution optical spectrum analyzer may be used at later stages tospeed up the spectrum acquisition process. High quality angle-polishedsingle-mode fibers will be used for connection to remove undesired lightintensity fluctuation due to the etalon effect. The data acquisitionprocess will be automated by a PC computer through Labview programs.

As discussed above, HQM-based sensors made of glass material haveadvantages over quartz, such as less susceptibility to mechanicaldefects and the capability of maintaining elasticity at pressures aboveseveral GPa. However, the manner in which the optical resonator isconstructed is of critical importance. This material system may not becapable of sustaining the high pressures that are required in thisapplication and it is important to determine the range over which thismaterial combination is stable, so that a more robust resonatorconfiguration can be identified in case of material failure.

In one embodiment, the optical resonant structure 30 can be constructedof a high refractive index chemical vapor deposition (CVD) glass that isdeposited on the surface of a silicon wafer (substrate) fortelecommunications applications. Differences in Young's modulus betweenthe optical resonant structure 30 and the substrate may induce crackingor delamination. Since the failures that would prove most detrimental tothe pressure sensor are those that will impact its optical properties,optical microscopy will serve well to characterize damage of interest.Structural testing of pressure sensors 14 may be accomplished by placingpressure sensors in pressure fluid and then taking the pressure fluid topressure. After cleaning, the pressure sensors will be stained with afluorophore and imaged on a confocal microscope. The transparency of theoptical resonant structure 30, the refractive index discontinuity at anycrack, and the 0.1 μm vertical step resolution of the availablemicroscope will permit a 3D image of internal defects. The pressuresensors 30 will then be stained for scanning electron microscopy by apreferential etch that will highlight any surface cracks, coated, andimaged.

Shown in FIG. 9 is an alternate embodiment of a pressure sensor 14 aconstructed in accordance with the present invention for use by thepressure sensing system 10 for determining a differential pressurebetween two vessels 200 and 202. The vessels 200 and 202 are pressurevessels holding two pressures where the differential pressure is to bemeasured.

The pressure sensor 14 a is provided with a substrate 32 a supportingthe optical resonant structure 30. The known light signal isevanescently coupled into the optical resonant structure 30 via theinput optical pathway 34. Light is evanescently coupled out of theoptical resonant structure 30 via the output optical pathway 36.Although not shown in FIG. 9, the pressure sensor 14 a can optionally beprovided with a cladding 40 a, similar to the cladding 40 for protectingthe optical resonant structure 30 or improving pressure sensitivity withthe cladding 40 formed of material of higher strain-optical effect.

The substrate 32 a is provided with a flexible membrane 206 upon whichthe optical resonant structure 30 is disposed. The membrane 206 isdeformed by pressure differentials between the vessels 200 and 202,which cause deformation of the optical resonant structure 30 and itsoptional cladding 40 a. The deformation of the optical resonantstructure 30 and its optional cladding 40 a cause a spectral shift whichis then used to calculate the pressure differential. If the pressurewithin one of the pressure vessels 200 and 202 is known, the pressuresensor 14 a can be used as an absolute pressure sensor.

The following references are expressly and specifically incorporated intheir entirety to the extent necessary to enable the invention.

References Cited

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The invention has been described by way of preferred embodiments.Various changes and modifications will be apparent to one skilled in theart to which it pertains. These modifications are intended to be withinthe spirit and scope of the invention defined by the following claims.

1. An optically-powered integrated microstructure pressure sensingsystem for sensing pressure within a cavity, the pressure sensing systemcomprising: a pressure sensor, comprising: an optical resonant structuresubject to the pressure within the cavity and having physical propertieschanging due to changing pressures within the cavity; a substratesupporting the optical resonant structure; an input optical pathwaypositioned to evanescently couple light transmitted through the inputoptical pathway into the optical resonant structure; and an outputoptical pathway positioned to collect light from the optical resonancestructure; a light source delivering a known light input into the inputoptical pathway whereby the known light input is evanescently coupledinto the optical resonant structure by the input optical pathway and aportion of such light is collected from the optical resonant structureby the output optical pathway; a light detector receiving the portion ofthe light collected from the optical resonant structure by the outputoptical pathway, and generating a light signal indicative of suchportion of the light collected from the optical resonant structure; atemperature compensation sensor generating a temperature signalindicative of the temperature near the optical resonant structure; and aspectrum detection device receiving the light signal and temperaturesignal, the spectrum detection device analyzing the light signal and thetemperature signal with a detection algorithm to generating a pressuresignal indicative of the pressure within the cavity.
 2. The pressuresensing system of claim 1, wherein the optical resonant structure has aQ-factor in a range greater than 1,000.
 3. The pressure sensing systemof claim 1, wherein the optical resonant structure is formed by aclose-looped wave-guiding structure.
 4. The pressure sensing system ofclaim 3, wherein the close-looped wave-guiding structure is defined by aring structure.
 5. The pressure sensing system of claim 1, wherein theoptical resonant structure is at least partially surrounded by anelastic cladding material positioned to transmit the pressure within thecavity to the optical resonant structure.
 6. The pressure sensing systemof claim 5, wherein the elastic cladding material is formed of anoptically transparent material.
 7. The pressure sensing system of claim5, wherein a refractive index of the elastic cladding material has anopposite temperature dependency to that of the optical resonantstructure to reduce the temperature dependency of the optical resonantstructure.
 8. The pressure sensing system of claim 5, wherein theelastic cladding material has a higher strain-optical effect than thatof the resonator to increase the pressure sensitivity of the opticalresonant structure.
 9. The pressure sensing system of claim 8, whereinthe optical resonant structure has a higher refractive index than theelastic cladding material.
 10. The pressure sensing system of claim 5,wherein the elastic cladding material comprises a coating covering theoptical resonant structure and bonded to the substrate.
 11. The pressuresensing system of claim 10, wherein the coating is formed of a polymericmaterial.
 12. The pressure sensing system of claim 5, wherein theelastic cladding material comprises a fluid positioned in contact withan exterior surface of the optical resonant structure.
 13. The pressuresensing system of claim 1, wherein a cross section dimension of theoptical resonant structure is comparable to a wavelength of the knownlight signal, and wherein the known light input propagating inside theoptical resonant structure establishes the optical resonance.
 14. Thepressure sensing system of claim 1, wherein the optical resonantstructure is micro-machined.
 15. The pressure sensing system of claim 1,wherein the light propagating inside the optical resonant structure andwherein the spectral information of the light propagating inside theoptical resonant structure is used by the spectrum detection algorithmto generate the pressure signal.
 16. The pressure sensing system ofclaim 1, wherein the portion of the light collected from the opticalresonant structure is indicative of interference between the lightpropagating inside the optical resonant structure and the known lightsignal in the input optical pathway.
 17. The pressure sensing system ofclaim 1, wherein the light source comprises a tunable narrow-band lightsource, and wherein the wavelength of the known light signal at theinput optical pathway is dithered across an interrogation wavelengthrange, the light intensity of the light traveling through the outputoptical pathway is recorded and then the spectrum is acquired.
 18. Thepressure sensing system of claim 1, wherein the input optical pathwayincludes a waveguiding structure.
 19. The pressure sensing system ofclaim 1, wherein the input optical pathway includes a free-space lightbeam.
 20. The pressure sensing system of claim 1, wherein the outputoptical pathway includes a waveguiding structure.
 20. The pressuresensing system of claim 1, wherein the output optical pathway includes afree-space light beam.
 21. The pressure sensing system of claim 1,wherein the substrate is defined as a deformable membrane which deformssubject to the pressure in the cavity, and wherein the optical resonantstructure is mounted onto the substrate such that the deformations ofthe membrane are communicated to the optical resonant structure wherebythe optical resonant structure is subject to the pressure within thecavity.
 22. The pressure sensing system of claim 21, wherein themembrane separates two vessels.
 23. The pressure sensing system of claim22, wherein the pressure within one of the vessels is known by thespectrum detection device.
 24. The pressure sensing system of claim 1,wherein the temperature compensation sensor comprises: an opticalresonant structure mounted so as to not be subject to the pressurewithin the cavity; an input optical pathway positioned to evanescentlycouple light transmitted through the input optical pathway into theoptical resonant structure of the temperature compensation sensor; andan output optical pathway positioned to collect light from the opticalresonance structure of the temperature compensation sensor.
 25. Thepressure sensing system of claim 24, wherein the optical resonantstructure of the temperature compensation sensor is mounted to asubstrate which is different than the substrate of the pressure sensor.26. The pressure sensing system of claim 25, wherein the opticalresonant structure of the temperature compensation sensor is encompassedby a rigid cladding.
 27. The pressure sensing system of claim 24,wherein the optical resonant structure of the temperature compensationsensor is mounted to the substrate of the pressure sensor.
 28. Thepressure sensing system of claim 27, wherein the optical resonantstructure of the temperature compensation sensor is encompassed by arigid cladding.